Fiber optic fuel and liquid gauge having an open rigid &#34;J&#34; shaped tube

ABSTRACT

A fiber optic fuel or liquid level gauge is disclosed which determines the liquid level by measuring the amount of light loss due to evanescent transfer to the fuel or liquid. An optical fiber is supported in the fuel or liquid tank, with a first end adjacent the highest possible liquid level to be measured with a reflector at the first end. The optical fiber extends downwardly through the tank, and its second end is connected to a light source for injecting light into the fiber. The intensity of the input light is compared to the intensity of light reflected from the first fiber end, and the liquid level is calculated from the light loss. The gauge has very high reliability since it has no moving mechanical parts.

This is a division of application Ser. No. 07/484,295 filed Feb. 23,1990, now U.S. Pat. No. 5,077,482.

BACKGROUND OF THE INVENTION

The present invention relates to fuel and liquid level gauges, and moreparticularly to a gauge which measures the fuel or liquid leveloptically by means of a fiber optic element.

Gauges for fuel and liquid tank levels are employed, for example, invehicles. The most common type of fuel gauge that is used at present isan electro-mechanical gauge. A float element which stays on the fuelsurface changes the position of a contact on a resistance wire as thefuel level changes. This is read out electrically and displayed on thevehicle dashboard. Such gauges are inaccurate and wear out with timebecause they have moving parts. The inaccuracies are aggravated by fueltanks which have convoluted configurations.

U.S. Pat. No. 3,995,168 describes an electro optical fluid measurementsystem for display of the level and specific density of a liquidcontained within a tank. The system employs a group of fiber opticbundles supported within a tank at a multiplicity of locations from thetop to the bottom of the tank. The level is detected by measuring lightfrom the respective groups of bundles to determine which bundle locationis exposed above the liquid level. To achieve a large number of possibleliquid level positions a corresponding number of fiber optic bundlesmust be used, thereby increasing the complexity and expense of thesystem.

It is therefore an object of the present invention to provide animproved fuel or liquid level gauge which is reliable and accurate.

A further object of the invention is to provide a fuel or liquid gaugewhich has no moving parts

Yet another object is to provide a fuel or liquid level gauge whichmeasures the liquid level optically.

SUMMARY OF THE INVENTION

A fiber optic liquid level gauge is provided in accordance with theinvention for measuring the level of liquid in a tank. The inventionexploits the evanescent wave loss from an optical fiber which occursfrom fiber/liquid interfaces. The gauge comprises an optical fiberdisposed within the tank and extending through the range of liquidsurface level positions to be measured by the gauge. The fiber ischaracterized by an inner fiber core and an outer fiber cladding, thethickness of fiber cladding on the fiber portions which extends throughthe range of positions being selected to provide significant evanescentwave loss at cladding/liquid interfaces.

The gauge further comprises a light source for injecting light into thefiber in order to measure the liquid level. A measuring means isprovided for measuring the degree of light intensity loss due toevanescent wave loss, and providing an intensity loss signal indicativeof the light intensity loss. A liquid level indicating means responsiveto the intensity loss signal indicates the liquid level in the tank.

The measuring means is calibrated to the particular tank and opticalfiber configuration. In a preferred embodiment, the measuring meansmeasures the intensity of light injected into the fiber, and measuresthe intensity of light which has traversed the fiber at least once, anddevelops. an intensity ratio signal which is the ratio of the lightintensity traversing the fiber to the incident light intensity. Theratio is indicative of the evanescent wave loss to the liquid and,therefore, provides a measure of the liquid level in the tank.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome more apparent from the following detailed description of anexemplary embodiment thereof, as illustrated in the accompanyingdrawings, in which:

FIG. 1 is a diagrammatic view illustrative of the operation of Snell'sLaw, a characteristic which is exploited by the invention.

FIG. 2 is a diagrammatic view illustrating the evanescent light wavewhich propagates parallel to the interface between an optic fiber andits surrounding media.

FIG. 3 is a simplified block diagram of a liquid level gauge embodyingthe invention.

FIG. 4 illustrates an alternate embodiment of an optic fiber arrangementfor a gauge embodying the invention.

FIG. 5 illustrates a second alternate embodiment of an optical fiberarrangement for a gauge embodying the invention.

FIG. 6 illustrates an optical fiber having a periodically varyingcladding thickness, which fiber may be employed in a gauge as shown inFIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention is a liquid or fuel level guage which measures the liquidlevel by means of an optical fiber. The invention exploits theevanescent wave losses in an optical fiber which take place only whenthe fiber is submerged in the liquid, i.e., only at fiber/liquidinterfaces. When the fiber is in air, the losses are minimal. Therefore,by disposing an optical fiber in the tank holding the liquid and bymeasuring the intensity of light which traverses the optical fiber, theposition of the fuel level is obtained.

Principle of Operation

At the boundary of two dielectrics with respective indexes of refractionof n₁ and n₂, some light is refracted and some is reflected. By Snell'sLaw the corresponding angle α₁ and α₂ of reflection, and the indexes ofrefraction are related by ##EQU1##

When the beam goes from an optically denser medium to an opticallythinner medium, there is a maximum angle α₂ for which α₁ becomes equalto 90° . This is the angle of total internal reflection. In this caseall light is reflected back into medium n₂. However, in a thin layer ofdielectric medium of refractive index n₁, on the boundary there is anexponentially decreasing intensity of light that propagates parallel tothe interface; this is known as the evanescent wave.

The light intensity distribution close to the interface A between thetwo media is shown in FIG. 2. The intensity of the evanescent wave inthe medium of refractive index n₁ is given (as a function of thedistance x from the interface) by eq. 1.

    I=I.sub.o e.sup.-βx                                   (1)

where the attenuation coefficient is (for a small glancing angle 90°-α₂): ##EQU2##

Here λ is the wavelength of light and Δn=n₂ -n₁ (n≃n₁ ≃n₂). Forpresently available communication fiber the attenuation distance is˜5μm. If one places a liquid surface at B (FIG. 2) with index ofrefraction higher than n₁, part of the radiation leaks through the layerbetween A and B (the tail part of the exponential decay). In order toexploit the light leakage in a liquid level gauge, the distance betweenA and B (that is, the fiber cladding thickness) should be selected suchthat a substantial portion of the initial intensity I_(o) leaks out overthe fiber length l (fuel depth).

The Preferred Embodiments

FIG. 3 shows the components of a fiber optic liquid level gaugeemploying the invention. The fuel or liquid is contained within tank 52,which may, for example, be a vehicle fuel tank. The optical fiber 54 ismounted within the tank 52 such that a first fiber end 56 is supportedat an upper level within the tank 52 at least as high as the highestliquid level to be measured by the gauge. The fiber may, for example, besupported within the tank by gluing it to the inner surface of the tank.A fiber end reflector 58 is disposed at a first end 56 of the fiber 52.The fiber 54 extends past a lower level within the tank 52 at least aslow as the lowest liquid level to be measured by the gauge. The opticalfiber is passed outside the tank to structure 62 which receives thesecond fiber end 60. A light source 64 such as a semiconductor laser isemployed to inject light into the second end 60 of the optical fiber 54.A fiber optic beamsplitter 66 is employed in cooperation withphotodetectors 68 and 70. The beamsplitter 66 functions to split off aportion of the light energy injected into the second fiber end 60 by thelight source to the photodetector 68, and to split off a portion of thelight energy which has traversed the optical fiber 54 and been reflectedby the reflector 58, and thus traversing the fiber length twice. Theoutputs of the photodetectors 68 and 70 are coupled to a leveldetermining circuit 72, which provides an output controlling the liquidlevel indicator device 74. The circuit 72 is calibrated to theparticular tank size and optical fiber, such that a given intensityratio value is known to correspond to a particular liquid level. Thelevel indicator may be an analog indicator or provide a digital readout.

Fiber beamsplitters suitable for use as beamsplitter 66 are commerciallyavailable; for example, the model F506B beamsplitter marketed by NewportResearch Corporation, 18235 Mt. Baldy Circle, Fountain Valley,California 92728-8020, is suitable for the purpose.

Photodetectors suitable for use as photodetectors 68 and 70 arecommercially available; for example, the model C30808 photodetectordevice marketed by RCA, 773 Donegal Business Center, P.0. Box 540, Mt.Joy, Pennsylvania 17552, is suitable for the purpose.

The light source 64 may comprise a semiconductor laser such as the modelLB1-02 laser marketed by Stantel Components, Inc., 636 Remington Road,Schaumberg, Illinois 60173. Alternatively, other light sources may beused, such as an incandescent light bulb or LED.

The optical fiber should be designed to provide appreciable losses dueto evanescent mode propagation over the length of the fiber atfiber/liquid interfaces. For example, assume the liquid is gasoline witha refractive index of about 1.57. An optic fiber comprising a corematerial of silicon dioxide (Si0₂) plus some germanium oxide (GeO₂) andhaving a refractive index of 1.46, and a cladding of pure quartz havinga thickness of a few microns with a refractive index of 1.45 may beused.

Various types of optical fibers can be employed with the inventionincluding thin cladding fibers, eccentric core fibers, periodicallyvariable cladding thickness fibers and fibers having double-layeredcladding.

The liquid level readout amounts to taking the ratio of thephotodetector currents I₂ and I_(l) at photodetectors 68 and 70. I₁gives the light intensity that has traversed the fiber twice. Thismethod of readout I₂ /I₁ is independent of the intensity of the lightsource and of the coupling efficiency of light into the fiber. ThereforeI₂ /I₁ is a uniquely defined function of the height of the fuel level,i.e., the length of fiber in the fuel, since the evanescent losses takeplace only in the fuel.

A gauge in accordance with the invention is usable not just for fuelsbut for liquids in general (e.g., oil, toxic wastes, drugs, etc.).

If the liquid leaves some residue on the fiber, the light loss is not aunique measure of the fuel level. In some applications, as where theliquid whose level to be measured is gasoline, a thin coating (e.g., afew microns) of fluorinated ethylene polypropylene ortetrafluoroethylene, such as that marketed by DuPont under theregistered trademark "Teflon," applied to the optical fiber may preventthe formation of residue on the fiber. Such a thin coating could beapplied, for example, by sputtering techniques. However, if theformation of residue is a problem for a particular application, thefiber can be surrounded by a flexible sleeve or membrane that contains aclean liquid, the surface height of which will then correspond to (or beproportional to) the surface height of the fuel or other liquid outsidethe sleeve. The clean liquid should be selected such that its surfacetension does not wet the optical fiber.

FIG. 4 illustrates such a flexible membrane employed with a fiber opticliquid level gauge employing the invention. Here the tank 100 containsthe liquid 110. The optical fiber 102 is brought in from the top of thetank, and extends downwardly to adjacent the bottom of the tank. Thelower end of the fiber 102 is terminated by a fiber end reflector 104.The fiber 102 extends through the top of the tank to the light injectionand detection elements as described above regarding the embodiment ofFIG. 3. Within the tank 100, the fiber 102 is disposed within a flexiblemembrane 106. A clean liquid 108 is disposed within the membrane 106. Ifthe fiber cladding is "Teflon," for example, a suitable liquid for useas the clean liquid 108 is glycerin. The membrane 106 may comprise, forexample, a fluoro-elastomer such as that sold under the registeredtrademark "Viton" by DuPont Automotive Products, 950 Stephenson Highway,P.0. Box 7013, Troy, Michigan 48007, having a thickness of about 0.001inch. The height, H1, of the liquid 110 is measured from the bottom 111of container 100 to the top 112 of liquid 110. Height H2 may bedetermined from height H2 of clear liquid 108 as measured from thebottom 107 of membrane 106 to the top 109 of liquid 108. The height H₁of the liquid 110 is related to the height H₂ of the clear liquid 108within the membrane 106 by the ratios of the respective densities of thetwo liquids. The force or pressure exerted by the liquid 110 against theflexible membrane 106 will be balanced by the force exerted by the clearliquid 108 against the flexible membrane. Thus H₁ /D₁ =H₂ /D₂, where D₁and D₂ are the respective densities of the liquids 110 and 108. Theheight H₂ is proportional to H₁, D₁ and D₂. It is not necessary that themembrane 106 be flexible throughout its length. For example, only arelative short segment of the membrane adjacent the bottom surface ofthe tank need be flexible; the remainder of the membrane element 106could be fashioned from a rigid tube. Thus, the gauge will measure thelevel of the liquid 108 within the membrane 106, which is in turnindicative of the level of the liquid 110 within the tank 100.

FIG. 5 illustrates a second alternate embodiment of a liquid level gaugeemploying the invention for use in measuring the level of liquid whichmay leave a residue on the fiber. Here the tank 150 contains a liquid152 whose level is to be measured by the gauge. The optical fiber 154 inthis embodiment is disposed within a rigid "J" shaped tube 185, open ateach end thereof. A clean liquid 160, such as metholyne iodide ormercury, is disposed within the tube 158 and has a higher density thanthe liquid 152. The fiber end 155 is terminated with a fiber reflector156.

The level 161 of the liquid 160 within the tube 158 will respond topressure from the liquid 152 in the tank at the liquid 152 - liquid 160interface within the tube 158, so that the level 161 of the liquid 160within the tube will be proportional to (although not necessarily equalto) the level of the liquid 152 within the tank 150. The output from thelevel detecting circuit (not shown in FIG. 5) can be calibrated so as toprovide proper level indicating signals to the level indicator (notshown in FIG. 5). To restrict undersired flow of the liquid 160 due tomovement of the tank, e.g., when mounted within an automobile, acapillary or narrow channel (not shown in FIG. 5) may be formed in thetube 158 close to the interface of liquid 152 and liquid 160. It willalso be appreciated that liquid 160 can be restrained from flowing fromtube 158 by selection of fluids which do not readily mix, appropriateselection of the length of the lower portion of the J tube, properselection of the liquid densities and other such parameters, dependingon the application. For example, where liquid 160 is mercury and liquid152 is gasoline, the chemistry of these liquids is such that they do notreadily mix. Also, the heavier mercury will tend to stay in the tubewith respect to the lighter gasoline. The gasoline will also exert adownward pressure on the mercury which will lower the level of themercury in the crook portion of the tube and also tend to keep themercury in the tube.

Also, in the case of these liquids, as see in FIG. 5, the lower portionof the J tube should have a sufficient upturn or crook such that theheavier mercury will not flow up and out of the open end of the J tubenear the reflector 156. It can be readily appreciated that mercury,having a density over 10 times that of gasoline, will tend to run out ofthe open end of the bottom of the tube if the tube has little or noupturned portion (ie approaches a straight tube). Also it can beappreciated that if the density of liquid 160 inside the tube is too lowrelative to the liquid 152, the heavier outside liquid will have astronger tendency to enter the tube and displace liquid 160.

FIG. 6 illustrates an optical fiber 200 having a cladding thicknesswhich is periodically variably, i.e., the fiber core 202 is surroundedby a fiber cladding 204 whose thickness varies periodically betweenregions of reduced thickness 206 and regions of increased thickness 208.The cladding thickness of the reduced regions 206 is selected to providesignificant evanescent mode losses when immersed in the liquid whoselevel is to be measured. The thickness of the increased regions 208 isselected so that significant evanescent mode losses do not occur inthese regions. The guage will thus give a characteristic reading whenthe liquid level is in one of the reduced regions 206, which readingwill not change until the liquid level moves to another one of thereduced regions 206. By calibrating the readings from the fiber 200, itwill thus be possible to know exactly in which reduced regions 206 theliquid level currently reposes and thus the height of the liquid. Theguage will be accurate only to a certain approximation, however, thegauge can easily provide accuracy to within 1%, which is much betterthan most conventional automobile gauges. For example, for a gas tank of10 inches in depth, accuracy to within 1% can be obtained by making theregions periodical over 1/10 of an inch. Such variations are quite longin terms of the wavelengths involved and the thickness of the fiberitself (<1mm). As a result, when an optical fiber 200 is employed in aliquid level gauge as illustrated in FIGS. 3, 4, or 5, the measuredintensity ratio from the gauge photodetectors will have discrete steppedvalues giving a level reading which is one of a plurality of possiblediscrete values.

Accuracy

For cubic or parallelepiped shaped gas tanks the ratio I₂ /I₁ variesexponentially with the fuel height. Other fuel container geometries haveto be calibrated individually. The limit of readout accuracy is set bythe photon noise (shot noise). For example, with a 30μWatt light sourceone could determine the height of the fuel level in a 1 meter high tankwith ˜0.1 mm accuracy. However, a tilt in a non-stationary gas tank willintroduce an error Δh in the fuel height (h) equal to ##EQU3## where αis the tilt angle. For α=10° the error is equal to 1.5 percent. This canbe reduced (by at least an order of magnitude) by having two fibergauges on opposite sides of the tank. When the vehicle (that containsthe fuel tank) is accelerated the surface of the liquid is tilted by anamount equal to where a is the acceleration of the vehicle and g isgravitational acceleration (10m/sec²). For example, if a car acceleratesfrom zero to 60 miles/hour in 10 seconds the acceleration a is

    a=2.7m/sec.sup.2

So the angle α is ##EQU4## and the readout error is ##EQU5##

This can again be reduced, if desired, by using two readout fibers. Ofcourse, the fuel level in a moving vehicle will have waves on it and,therefore, the readout will have to be averaged over a period of a fewseconds (depending on the desired accuracy).

It is understood that the above-described embodiments are merelyillustrative of the possible specific embodiments which may representprinciples of the present invention. Other arrangements may readily bedevised in accordance with these principles by those skilled in the artwithout departing from the scope of the invention.

What is claimed is:
 1. A fiber optic liquid level guage for measuringthe level of a first liquid in a container comprising:an optical fiberdisposed within the container and extending through the range of liquidlevel positions to be measured by the guage, wherein the optical fiberis characterized by an inner fiber core and an outer fiber cladding, thethickness of fiber cladding on the fiber portion which extends throughsaid range of positions being selected to provide significant evanescentwave loss when the cladding is immersed in the liquid; a light sourcefor injecting liquid into said fiber; measuring means for measuring thedegree of light intensity loss over the fiber length due to evanescentwave loss at the fiber/liquid interface, and providing an intensity losssignal indicative of the light intensity loss; liquid level indicatingmeans responsive to said intensity loss signal for indicating the liquidlevel in the container; and an open rigid "J" shaped tube disposedwithin the container, said optical fiber being supported within thetube, and said tube further containing a second liquid having a densitygreater than the density of said first liquid, said first liquid in saidtank exerting pressure on said second liquid to define a surface levelin said tube, said optical fiber for providing an intensity loss whichis a function of said surface level which intensity loss is therebyindicative of the level of said first liquid in the container.
 2. Afiber optice liquid level guage for measuring the level of a firstliquid in a container comprising:an optical fiber disposed within thecontainer and extending through the range of liquid level positions tobe measured by the guage, wherein the optical fiber is characterized byan inner fiber core and an outer fiber cladding, the thickness of fibercladding on the fiber portion which extends through said range ofpositions being selected to provide significant evanescent wave losswhen the cladding is immersed in the liquid; a light source forinjecting light into said fiber; measuring means for measuring thedegree of light intensity loss over the fiber length due to evanescentwave loss at the fiber/liquid interface, and providing an intensity losssignal indicative of the light intensity loss; and liquid levelindicating means responsive to said intensity loss signal for indicatingthe liquid level in the container, wherein said fiber cladding ischaracterized by a thickness which varies periodically from regions ofreduced thickness to regions of increased thickness, the reducedthickness being selected such that there is significant evanescent waveloss at cladding/liquid interface in the reduced regions, and theincreased thickness is selected such that there is no significantevanescent wave loss at cladding/liquid interfaces in the increasedregions, whereby said intensity loss signal is characterized by a numberof possible discrete values.
 3. A fiber optic liquid level guage formeasuring the level of a first liquid in a container comprising:anoptical fiber having a first end and a second end disposed in saidcontainer and extending within the container through the range of liquidsurface levels to be measured, the optical fiber characterized by aninner fiber core and an outer fiber cladding, the thickness of the fibercladding being selected to provide significant evanescent wave loss atfiber/liquid interfaces; a fiber end reflector at said first end of saidoptical fiber; a light source for injecting light into said second endof said fiber; a first optical coupler for coupling off a portion of thelight being injected into the optical fiber; a second optical coupleradjacent said second end for coupling off a portion of the light whichhas traversed the fiber and been reflected form the optical reflector atthe first end; means for comparing the light intensity from the firstoptical coupler with the light intensity form the second optical couplerand determining the liquid level; and an open rigid "J" shaped tubedisposed within the container, said optical fiber being supported withinthe tube, and said tube further containing a second liquid having adensity greater than the density of said first liquid, said first liquidin said tank exerting pressure on said second liquid to define a surfacelevel in said tube, said optical fiber for providing an intensity losswhich is a function of said surface level which intensity loss isthereby indicative of the level of said first liquid in the container.4. A fiber optic liquid level guage for measuring the level of a firstliquid in a container comprising:an optical fiber having a first end anda second end disposed in said container and extending within thecontainer through the range of liquid surface levels to be measured, theoptical fiber characterized by an inner fiber core and an outer fibercladding, the thickness of the fiber cladding being selected to providesignificant evanescent wave loss at fiber/liquid interfaces; a fiber endreflector at said first end of said optical fiber; a light source forinjecting light into said second end of said fiber; a first opticalcoupler for coupling off a portion of the light being injected into theoptical fiber; a second optical coupler adjacent said second end forcoupling off a portion of the light which has traversed the fiber andbeen reflected form the optical reflector at the first end; and meansfor comparing the light intensity from the first optical coupler withthe light intensity from the second optical coupler and determining theliquid level; wherein said fiber cladding is characterized by athickness which varies periodically from regions of reduced thickness toregions of increased thickness, the reduced thickness being selectedsuch that there is significant evanescent wave loss at cladding/liquidinterfaces in the reduced regions, and the increased thickness isselected such that there is no significant evanescent wave loss atcladding/liquid interfaces in the increased regions, whereby saidintensity loss signal is characterized by a number of possible discretevalues.
 5. An optical fiber comprising:a fiber core; and a fibercladding, wherein said fiber cladding is characterized by a thicknesswhich varies periodically from regions of reduced thickness tofacilitate evanescent wave loss from said fiber in the reduced regionsto regions of increased thickness to substantially prevent evanescentwave loss from said fiber in the increased regions.